Lithium ion secondary battery and its negative electrode

ABSTRACT

The present invention relates to a lithium ion secondary battery using as the negative active material thereof a carbon material that enables the absorption and release of lithium ions and has volume resistivity not exceeding 5.0×10 −3  ohmcm, thereby preventing battery temperatures from increasing abruptly due to low volume resistivity of the carbon material even when short-circuiting takes place inside of the battery.

THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCTINTERNATIONAL APPLICATION PCT/JP97/02778.

FIELD OF THE INVENTION

The present invention relates to a lithium ion secondary battery and acarbon material for the negative electrode used in the lithium ionsecondary battery.

BACKGROUND OF THE INVENTION

As many more types of electronic equipment are being produced inportable versions and cordless versions in recent years, demands forsecondary batteries, each having a small size, a light weight and a highenergy density and serving as a power supply for the foregoingelectronic equipment have been increasing.

In this regard, a nonaqueous electrolyte secondary battery, particularlya lithium ion secondary battery, has great expectations as a batteryhaving a high voltage high energy density.

So far, a battery family employing transition metal oxides or sulfidessuch as manganese dioxides, molybdenum disulfides and the like to serveas a positive electrode and metallic lithium or alloys of lithium as anegative electrode has been put forth to produce lithium ion secondarybatteries.

However, when metallic lithium is used as the negative electrode, themetallic lithium is deposited on the negative electrode in a needle-likeshape or a moss-like configuration during a charge, piercing throughseparators and coming into contact with the positive electrode, therebycausing serious problems in safety of the battery such as a sudden risein battery temperatures and the like due to internal short-circuiting.

As a result, use of a carbon material that can absorb and releaselithium ions as the negative electrode has been proposed. In this case,lithium ions get into the carbon material between the layers thereof andno lithium is deposited on the negative electrode, thus eliminating thedangers of degrading the safety of the battery and at the same timecontributing to an improvement in rapid charge characteristics. Becauseof these reasons, lots of R&D activities are being carried out atpresent in this particular area.

In these cases, a lithium-containing metal oxide such as LiCoO₂, LiNiO₂or the like is used as the material for negative electrodes.

When such an accident as a battery being pressed strongly from both sidesurfaces occurs, however, the external pressure applied to the sidesurfaces of the battery used to cause the positive and negativeelectrodes to contact with each other after breaking through separators,resulting in development of an internal short circuit.

Upon developing the internal short circuit as described in the above,large currents flowing through the areas where the positive and negativeelectrodes are in contact with each other bring about a heat generationcaused by Joule's heat due to large contact resistance, thereby raisinga problem of an abrupt increase in battery temperatures.

DISCLOSURE OF THE INVENTION

The present invention deals with the problem as described in the aboveand has an objective of providing much safer types of lithium ionsecondary batteries, temperatures of which do not increase abruptly evenif the batteries are crushed.

In order to accomplish this objective, the present invention hasproposed a negative electrode for lithium ion secondary batteries thatuses a material containing a carbon material of less than 5.0×10³¹ ³ohmcm in volume resistivity, whereby absorption and release of lithiumions are made possible. Therefore, even when the batteries are crushedand internal short-circuiting takes place, generation of Joule's heat issuppressed due to small volume resistance of carbon material.

Further, the present invention has disclosed a lithium ion secondarybattery using a lithium-containing transition metal composite oxide asthe positive electrode thereof and also using as the negative electrodethereof a carbon material that can absorb and release lithium ions andwherein volume resistivity is made less than 5.0×10⁻³ ohmcm.

Furthermore, the present invention has disclosed use of a negativeelectrode wherein filling density of a carbon material ranges from 1.2to 2.0 g/cc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a lithium ion secondary battery inan exemplary embodiment of the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

Next, an explanation will be made on a lithium ion secondary battery inan exemplary embodiment of the present invention, which contains a groupof electrodes formed by winding a positive electrode and negativeelectrode together with a separator.

According to the present invention, it is preferred that volumeresistivity of carbon material is made smaller than 5.0×10³¹ ³ ohmcmbecause larger resistance of the carbon material than the above bringsabout generation of large Joule's heat, when the battery is crushed, theseparator is broken and the positive electrode and negative electrodecome into contact with each other, large electric currents concentrateinto contact points, thereby causing sometime the battery temperaturesto rise locally and abruptly.

The volume resistivity of an electrode that uses a carbon materialchanges according to the filling density of carbon per unit volume ofthe electrode.

Since the energy density of a battery is determined by how much ofelectrode materials is packed in a limited volume available inside abattery container, it is advantageous to have the most possible fillingdensity. With a large filling density, volume resistivity tends todecrease resulting in a favorable performance of the battery but whenthe filling density exceeds a certain limit the electrodes become lessporous and infiltration of electrolyte into the electrodes becomes moredifficult with a resulting increase in mobility resistance anddeterioration in battery's high rate characteristics.

Therefore, it is preferred that the filling density of carbon pernegative electrode volume is made less than 2.0 g/cc. On the other hand,when the filling density is too small, the volume resistivity increasesand at the same time the battery capacity is reduced. Therefore, it ispreferred that the filling density is made more than 1.2 g/cc.

The nonaqueous electrolyte used in a lithium ion secondary battery ofthe present invention is prepared by dissolving an electrolyte in anonaqueous solvent.

As the nonaqueous solvent, an organic solvent generally employed in alithium ion secondary battery can be used singularly or as a combinationof several kinds thereof.

For example, such cyclic carbonates as ethylene carbonate (EC),propylene carbonate (PC), buthylene carbonate (BC) and the like, suchchain carbonates as dimethyl carbonate (DMC), diethyl carbonate (DEC),ethylmethyl carbonate (EMC) and the like and such aliphatic carboxylicacids such as methyl propionate, ethyl propionate and the like arepreferred.

Particularly, mixtures of cyclic carbonates and chain carbonates ormixtures of cyclic carbonates, chain carbonates and aliphatic carboxylicacid esters are more preferred.

As the electrolyte, for example, such lithium salts as lithium/perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumfluoroborate (LiBF₄), lithium arsenic hexafluoride (LiAsF₆), lithiumtrifluorosulforate (LiCF₃SO₃), bis(trifluoromethyl)sulfonylimido lithium[LiN(CF₃SO₂)₂] and the like can be used in isolation from others or incombination with some of others.

Particularly, use of lithium hexafluorophosphate (LiPF₆) is preferred.

The amount of electrolyte dissolution in nonaqueous solvent ranges from0.2 mol/l to 2 mol/l and the amount of 0.5 mol/l to 1.5 mol/l isparticularly preferred.

As the positive electrode active substance employed in a lithium ionsecondary battery of the present invention, a variety oflithium-containing transition metal oxides (lithium manganese doubleoxides such as LiMn₂O₄ and the like, lithium-containing nickel oxidessuch as LiNiO₂ and the like and lithium-containing cobalt oxides such asLiCoO₂, and the foregoing oxides wherein part of manganese, nickel andcobalt is replaced with other transition metals and the like or alithium-containing vanadium oxide and the like) and chalcogen compounds(such as manganese dioxide, titanium disulfide, molybdenum disulfide andthe like) can be used. Particularly, use of lithium-containingtransition metal oxides is preferred.

As the conductive material for positive electrode, artificial graphite,carbon black (such as acetylene black and the like) or nickel powder andthe like can be used.

Further, as the carbon material employed in a lithium ion secondarybattery of the present invention for absorbing or releasing lithiumions, what is obtained by sintering organic polymer compounds (such asphenol resin, polyacrylonitrile, cellulose and the like), or what isobtained by sintering coke and pitch or graphite that includesartificial graphite and natural-graphite-can be used. Particularly, useof graphite materials that include what is obtained by applyinghigh-temperature processing to meso-phase spherical particles obtainedby sintering meso-phase pitch, artificial graphite, natural graphite andthe like is preferred.

Next, an explanation will be made on some of the exemplary embodimentsof the present invention with reference to drawings.

FIG. 1 shows the structure of a cylindrical battery that has beenprototyped to show the effectiveness of the exemplified embodiments ofthe present invention.

This prototyped battery measures 20 mm in diameter and 70 mm in totalheight.

EXEMPLARY EMBODIMENT 1

A positive electrode 1 in FIG. 1 is prepared by first applying apaste-like mixture, which comprises lithium cobalt oxide (LiCoO₂)serving as the active material and produced by sintering a mixture oflithium carbonate (Li₂CO₃) and 4,3-cobalt oxide (Co₃O₄) at 900° C. inthe air, having then acetylene black mixed thereto by 3 weight % to makea conductive agent and finally having an aqueous dispersion system ofpolytetrafluoroethylene resin blended therewith by 7 weight % to serveas a binder, to both surfaces of a core material formed of aluminum foiland then, after drying and rolling, by cutting out to a dimension of 57mm wide and 520 mm long. A positive electrode lead tab 4 is spot-weldedto the end of the positive electrode 1.

A negative electrode 2 is prepared by applying a paste-like mixture,which comprises artificial graphite (average particle size of 25 μm indiameter) as an active material with styrene-butadiene-rubber serving asa binder blended with the active material by 5 weight % and then iscompleted by having the foregoing mixture kept in suspension in anaqueous solution of carboxymethylcellulose, to both surfaces of a corematerial formed of copper foil and then, after drying, by rolling tomake the packing density of carbon equal to 1.4 g/cc and by cutting outto a dimension of 0.2 mm thick, 59 mm wide and 550 mm long.

A negative electrode lead tab 5 is spot-welded to the end of thenegative electrode 2.

A separator 3 is a porous film formed of polypropylene resin andprepared by cutting to a dimension that is larger in width than thepositive electrode 1 and negative electrode 2.

Next, the foregoing positive electrode 1 and negative electrode 2 arewound together with the separator 3 sandwiched in between so that thecross-sectional view of the wound body shows spiral patterns, thuscompleting a bundle of electrodes.

Then, a lower insulating ring 6 is placed on the bottom side of theelectrode bundle, the whole of this is contained in a battery case 7that measures 20 mm in diameter and 70 mm in height, and the negativeelectrode lead tab 7 is spot-welded to the battery case 7.

An upper insulating ring 8 is placed on the top side of the electrodebundle and a groove is formed on the surfaces of the battery case 7 inthe upper part thereof, and then a nonaqueous electrolyte is filled inthe battery case 7.

The electrolyte has been prepared by mixing ethylene carbonate (EC) anddimethyl carbonate (DMC) to 1:1 ratio in volume and then dissolving theforegoing mixture in lithium hexafluorophosphate (LiPF₆) of 1 mol/l.

The positive electrode lead tab 4 is spot-welded to a sealing plateassembly 9 which has a gasket built-in in advance and then the sealingplate assembly 9 is put together with the battery case 7, thuscompleting a battery A for the present exemplary embodiment.

EXEMPLARY EMBODIMENT 2

Another battery is prepared in the same way as was in ExemplaryEmbodiment 1 except for having used artificial graphite of 15 μm inaverage particle size as the negative electrode material, thuscompleting a battery B for Exemplary

Embodiment 2.

EXEMPLARY EMBODIMENT 3

Still another battery is prepared in the same way as in ExemplaryEmbodiment 1 except for having made the density of the negativeelectrode material equal to 2.0 g/cc, thus completing a battery C forExemplary Embodiment 3.

EXEMPLARY EMBODIMENT 4

Still another battery is prepared in the same way as in ExemplaryEmbodiment 1 except for having made the density of the negativeelectrode material equal to 1.2 g/cc, thus completing a battery D forExemplary Embodiment 3.

COMPARISON EXAMPLE 1

A battery is prepared in the same way as was in Exemplary Embodiment 1except for having used artificial graphite of 6 μm in average particlesize as the negative electrode material, thus completing a battery E foruse in Comparison Example 1.

COMPARISON EXAMPLE 2

Another battery is prepared in the same way as in Exemplary Embodiment 1except for having used natural graphite of 20 μm in average particlesize as the negative electrode material, thus completing a battery F foruse in Comparison Example 1.

COMPARISON EXAMPLE 2.

Still another battery is prepared in the same way as in ExemplaryEmbodiment 1 except for having used natural graphite of 7 μm in averageparticle size as the negative electrode material, thus completing abattery G for use in Comparison Example 3.

COMPARISON EXAMPLE 4

Still another battery is prepared in the same way as in ExemplaryEmbodiment 1 except for having made the density of the negativeelectrode material equal to 2.1 g/cc, thus completing a battery H forComparison Example 4.

COMPARISON EXAMPLE 5

Still another battery is prepared in the same way as in ExemplaryEmbodiment 1 except for having made the density of the negativeelectrode material equal to 1.0 g/cc. However, since the amount ofcarbon material is reduced to realize a smaller density of carbonmaterial in the negative electrode, a sufficient initial capacity hasnot been gained.

Volume resistivity of each respective negative electrode material of theforegoing Exemplary Embodiments 1 to 4 and Comparison Examples 1 to 5 ismeasured by use of a four-terminal method with fine particle resistancemeasurement equipment (MCP-PD41 of Mitsubishi Chemical Co., Ltd.) and inaccordance with JIS-K7194.

When resistance of fine particles is measured, the pressure applied tothe fine particles has been adjusted so that each respective negativeelectrode material may realize the density as required of acorresponding negative electrode.

Prototyped batteries comprising 50 each of the batteries A, B, C and Dof the present invention and 50 each of the batteries E, F, G and H asprepared for comparison examples are put on a constant voltage constantcurrent charge under the conditions of a charge voltage of 4.2 V and acurrent limit of 800 mA at 20° C. for 2 hours, and then subjected to abattery crush test.

The battery crush test is conducted as follows:

Using a round rod of 6 mm in diameter in the shape of a cylindricalcolumn, press the round rod on to the middle side of a battery to betested in a direction perpendicular to the direction, in which thebattery extends over the longest dimension, till the thickness ofbattery is reduced to one half of the original thickness.

High-rate characteristics of each respective battery are investigated bytaking a ratio between the discharge capacity at a low current (200 mA)and the discharge capacity at a high current (2000 mA).

Table 1 shows the number of batteries that have ignited in the crushtest, volume resistivity of negative electrode carbon material, densityof negative electrode carbon material and capacity ratio between lowcurrent and high current for each respective test.

Table 1 tells that there is an explicit effect achieved by the batteryof the present invention when the ignition rate of the batteries A, B, Cand D in Exemplary Embodiments of the present invention is compared withthat of the batteries E, F, G and H in Comparison Examples.

The batteries A, B, C and D using negative electrode carbon materialswith volume resistivity of less than 5×10⁻³ ohmcm have not shown anyabrupt rise in battery temperatures even when the batteries aresubjected to crush tests.

On the other hand, the battery H of Comparison Example 4 has shown noabrupt increase in temperatures but the high-rate characteristicsthereof are found not good due to the fact that the density of thenegative electrode carbon material exceeds 2.0, thus provinginappropriate for a practical use.

TABLE 1 Volume Number of resistivity Density batteries of carbon ofcarbon Battery having ignited material material Capacity Code in crushtests (ohmcm) (g/cc) ratio Battery A 0/50 2.37 × 10⁻³ 1.40 0.95 BatteryB 0/50 4.33 × 10⁻³ 1.40 0.98 Battery C 0/50 2.27 × 10⁻³ 2.00 0.90Battery D 0/50 5.00 × 10⁻³ 1.20 0.95 Battery E 5/50 6.21 × 10⁻³ 1.400.95 Battery F 3/50 5.77 × 10⁻³ 1.40 0.90 Battery G 10/50  7.24 × 10⁻³1.40 0.92 Battery H 0/50 2.10 × 10⁻³ 2.20 0.40

Industrial Applicability

A lithium ion secondary battery of the present invention is producedusing a carbon material with volume resistivity not exceeding 5.0×10⁻³ohmcm. Therefore, even when the battery is crushed, much Joule's heat isnot generated, thereby enabling the prevention of an abrupt increase inbattery temperatures.

What is claimed:
 1. A negative electrode for a lithium secondarybattery, the electrode comprising a particulate carbon material enablingthe absorption and release of lithium ions and a binder; in which: theparticulate carbon material is artificial graphite; the particulatecarbon material has a volume resistivity not exceeding 5.0×10³¹ ³ohm·cm; and the filling density of the particulate carbon materialranges from 1.2 to 1.40 g/cc.
 2. The negative electrode of claim 1wherein the artificial graphite has an average particle size of 15 to 25μm.
 3. The negative electrode of claim 1 wherein the particulate carbonmaterial has a filling density of 1.4 g/cc.
 4. A lithium ion secondarybattery comprising: (a) a positive electrode comprising alithium-containing transition metal composite oxide; and (b) a negativeelectrode comprising a particulate carbon material enabling theabsorption and release of lithium ions, and a binder; in which: theparticulate carbon material is artificial graphite; the particulatecarbon material has a volume resistivity not exceeding 9 5.0×10³¹ ³ohm·cm; and the filling density of the particulate carbon materialranges from 1.2 to 1.40 g/cc.
 5. The lithium ion secondary battery ofclaim 4 additionally comprising a nonaqueous electrolyte.
 6. The lithiumion secondary battery of claim 5 wherein the transition metal oxide isselected from the group consisting of lithium manganese double oxides,lithium-containing nickel oxides, and lithium-containing cobalt oxides.7. The lithium ion secondary battery of claim 6 wherein the nonaqueouselectrolyte comprises a lithium salt selected from the group consistingof lithium perchlorate, lithium fluoroborate, lithium arsenichexafluoride, lithium trifluorosulfonate,bis(trifluoromethyl)sulfonylimido lithium, and lithiumhexafluorophosphate.
 8. The lithium ion secondary battery of claim 7wherein the transition metal oxide is lithium cobalt oxide.
 9. Thelithium ion secondary battery of claim 8 wherein the artificial graphitehas an average particle size of 15 to 25 μm.
 10. The lithium ionsecondary battery of claim 5 wherein the artificial graphite has anaverage particle size of 15 to 25 μm.
 11. The lithium ion secondarybattery of claim 4 wherein the particulate carbon material has a fillingdensity of 1.4 g/cc.
 12. The lithium ion secondary battery of claim 4wherein the artificial graphite has an average particle size of 15 to 25μm.